Primary systemic lactic acidosis represents a serious and often fatal disorder of several possible causes. During ischemia brain lactate levels rise precipitously (Siesjo, 1978, "Brain Energy Metabolism," John Wiley and Sons; 1981, J. Cereb. Flow Metab., 1:155-185; and references quoted therein). As discussed previously (Pulsineli, et al, 1982, Neurology, 32:1239-1246), brain lactate concentration is a function of several factors, including tissue oxygen tension (Gurdjian et al, 1944, Arch. Neurol. Psychiatry, 57:472-477), the glycolytic substrate supply from blood and brain (Ljunggren et al, 1974, Brain Res., 77:173-186; Siesjo, 1981, supra) and the rate of lactate efflux into the venous circulation (Zimmer and Lang, 1975, Am. J. Physiol., 229:432-437). Ljunggren et al, 1974, (supra) showed that, in the severely ischemic brain where the blood glucose oxygen supply approached zero and egress of lactate from brain is negligible, the tissue lactate concentration is proportional to cerebral stores of glycolytic substrate at the onset of the ischemic insult. In the four-vessel occlusion (4-VO) model of near-complete forebrain ischemia in the rat, lactate levels rise from a normal of .about.1.0 mM to as high as 13.0 mM in normoglycemic animals (Pulsinelli et al, 1983, J. Neurochem., 40:1500-1503; Kraig et al, 1985a, Brain Res., 342:281-290). Forebrain ischemia under normoglycemic conditions results in selective neuronal destruction with the sparing of other tissue elements (Pulsinelli et al, 1983, supra). In contrast, equivalent degrees and duration of ischemia delivered during hyperglycemia produce tissue infarction with necrosis of all cellular elements. In this 4-VO model, lactate levels rise to greater than 16 mM in the hyperglycemic animals (Pulsinelli et al, 1982, supra) and may reach as high as 31 mM in total brain ischemia induced by cardiac arrest (Kraig et al 1985a, supra). It has been proposed that lactic acidosis, and the attendant decrease in pH, may be a major cause of severe ischemic brain injury (Myers, 1979, Adv. Neurol., Vol. 26, Fahn et al, eds. pp. 195-213, Raven Press, N.Y.).
The relationship between brain lactate concentration and pH is complex and there is evidence of compartmentation of H.sup.+ and of the principal buffer, bicarbonate (Kraig et al, 1985a, supra; 1985b, Progress in Brain Res., Vol. 63, Kogure et al, eds. pp. 155-166: 1986, Am. J. Physiol., 250:R348-R357). Nevertheless, in cardiac arrest, the brain pH of normoglycemic rats drops from 7.2.+-.0.02 to 6.79.+-.0.02 (brain lactate=8-12 mM) whereas the brain pH of hyperglycemic rats drops to 6.19.+-.0.02. Furthermore, in the 4-VO model of forebrain ischemia, the brain pH falls to 6.0-6.2 during the 30-minute insult and abruptly falls still further on reperfusion, to a low of 5.4 if the rat is severely hyperglycemic (Kraig et al, 1985b, supra). It is possible that the continued production of lactate in the brains of ischemic hyperglycemic animals occurred mostly in the astrocytes with a theoretical pH drop in these cells to as low as 5.2 or lower (Kraig et al, 1986, supra). Finally, Nordstrom et al, (1978a, Stroke, 9:327-335: 1978b, Stroke, 9:335-343, and 1978c, J. Neurochem., 30:479-486) showed that incomplete cerebral ischemia (i.e. that accompanied by a small trickle of arterial blood) caused a rise of brain lactate that was twice that resulting from complete ischemia. The finding may explain in part the well-known observation that partial ischemia may be equally or more damaging to brain than similar durations of total ischemia (see also Kalimo et al, 1981, J. Cerebral Blood Flow Metab., 1:313-327; Rehncrona et al, 1981, J. Cerebral Blood Flow Metab., 1:297-311).
The weight of evidence favors lactic acidosis as a contributing factor to ischemic brain damage in normoglycemic animals; in hyperglycemic animals lactic acidosis may be the major contributing factor in converting selective neuronal damage into infarction during or following ischemia. The mechanism is unknown, but may be due in part to pH-induced changes in rates of reactions catalyzed by enzymes with narrow pH-activity profiles. Alternatively, Pulsinelli et al, (1985, Cerebrovascular Diseases, Plum and Pulsinelli, eds. pp. 201-210, Raven Press, N.Y.) have proposed that the lowered pH of ischemic brain favors the release and conversion of protein-bound Fe.sup.3+ to free Fe.sup.2+ ; the latter can then act as a source of highly reactive and damaging free radicals. Whatever the mechanism of the cell damage, a rational approach to minimizing ischemic damage to nervous tissue is to reduce lactic acid production as much as possible.
In addition to stroke, lactic acidosis is a serious complication in a number of general clinical settings in which (a) there is poor oxygen perfusion or (b) metabolism is disrupted by e.g. infections, hereditary causes, drug ingestion, liver damage, kidney damage or leukemia (for a discussion see Cohen and Woods, 1976, Clinical and Biochem. Aspects of Lactic Acidosis, Blackwell Sci. Pub., Oxford). Treatments have included sodium bicarbonate, trihydroxymethylaminomethane, electron acceptors (methylene blue), and glucose (with or without insulin): hemodialysis, peritoneal dialysis and O.sub.2 have been used as therapeutic adjuncts (Cohen et al, supra). More recently, dichloroacetate has been employed in the treatment of animals with experimentally-induced lactic acidosis and of acidotic patients (e.g. Curry et al, 1985, Clin. Pharmacol. Ther., 37:89-93 and references cited therein).
Apparently, dichloracetate exerts its effect by activating pyruvate dehydrogenase (through inhibition of the kinase of the complex), thereby accelerating the removal of pyruvate before it can be reduced to lactate (for a review see Crabb et al, 1981, Metabol., 30:1024-1039). However, dichloroacetate cannot prevent lactate buildup during ischemia because the conversion of pyruvate to acetyl CoA is an oxidative process. The best that dichloroacetate can accomplish is to hasten lactate removal following removal of the ischemic insult by which time tissue damage may have already occurred. Moreover, there is some evidence that dichloroacetate is neurotoxic, can cause cataracts and may be mutagenic. Finally, dichloroacetate interacts with many enzymes and is metabolized to toxic oxalate and highly reactive glyoxylate.
For the above reasons, it may be more advantageous to treat lactic acidosis (particularly that arising from an ischemic insult) with specific and reversible inhibitors of lactate dehydrogenase. The rationale is as follows. If lactate dehydrogenase is shut down, anaerobic glycolysis will halt because NAD.sup.+ necessary for the glyceraldehyde-3-phosphate dehydrogenase reaction will not be regenerated. This in turn means that ATP will not be generated from phosphoglycerate and pyruvate kinases for the hexokinase and phosphofructokinase reactions. In the case of an ischemic insult, withdrawal of the insult should result in reestablishment of normal aerobic metabolism in that tissue. Aerobic glycolysis in non-ischemic tissue should not be greatly affected by inhibition of lactate dehydrogenase. There is a precedent for such reasoning: For example, Friede and VanHoutte (1961, Exp. Neurol., 4:197-204) showed that cerebellar tissue slices undergo necrobiosis when cellular respiration is blocked selectively but not when respiration and glycolysis are blocked simultaneously. The problem then is the design of such a specific inhibitor of lactate dehydrogenase.